1. Field of the Invention
The present invention relates generaly to an instantaneous floating point amplifier and particularly to such an amplifier which includes offset voltage correction data processing capabilities.
2. Description of the Prior Art
A seismic receiver employs an instantaneous floating point (IFP) amplifier because of the tremendous dynamic range requirements in which it is used. That is, a near-channel signal is proportionally much stronger than a far-channel signal with the signals from the seismic detectors having various amplitudes in between. Therefore, the gain for the amplifier with a near-channel signal must be much lower than the gain for the same amplifier when employed with a relatively weak far-channel signal. It has been common practice to use a plurality of cascaded amplifier stages for the basis of developing two parts of a composite representation each time a signal is amplitude sampled and that amplitude is digitized.
First, the signal is applied to a sample-and-hold network, the output therefrom being sequentially test sampled for amplitude with different combinations of amplifiers until a given combination is reached that is greater than a predetermined threshold level. Each time a new combination of amplifiers is attempted, a digital gain word is developed for that combinatin. When the threshold level is exceeded in the sequential test sample sequence, the gain word is stored. The amplified value corresponding to this gain word, which is actually a multiplier factor, is applied to an output network as the analog mantissa value. The mantissa value is then digitized and processed in time coincidence with its related gain word as representative of the total value.
Because of the wide dynamic range of the amplifiers, as well as the thermal environmental operating conditions, there is an offset voltage that is inherent in the IFP. Furthermore, the offset voltage tends to change over a period of time as the amplifier heats up or cools down and for other reasons. The offset signal appears as a dc voltage superimposed on the analog data voltage and it can be substantial, in fact, having an even larger order of magnitude than a weak data signal. The prior art techniques for removing or compensating for the dc offset have mainly included filters. One technique that has been employed involves the measuring of the dc offset independently of signal, converting the value of dc offset to a digital value, shifting the developed two-part digitized data into a register and digitally correcting the data in the register by the digital offset voltage value. This is sometimes referred to as altering or "normalizing" the data. Although this technique accomplishes the desired results, the operation uses a large number of electronic components.
It is therefore, a feature of the present invention to provide improved offset voltage correction data processing in an instantaneous floating point amplifier without using a large number of digital logic components to perform the calculation.
It is another feature of the present invention to provide improved offset voltage correction data processing in an instantaneous floating point amplifier by determining the offset voltage correction utilizing components used for other operational purposes that are part of the analog-to-digital output network of the amplifier.